A critical function of the nervous system is to process sensory stimuli differentially according to their salience and behavioral importance. Little is known, however, about the mechanisms by which sensory responses are modulated in the brain. The barn owl has a highly evolved auditory localization system that is an excellent model for studying sensory processing. Preliminary experiments indicate that a midbrain cholinergic nucleus modulates responses in this system. The research proposed here will use electrical microstimulation to activate this nucleus, and address the following questions: 1) how does activation of this nucleus modulate neural responses to auditory and visual localization cues? 2) which neurotransmitter receptors mediate this modulation? This research will contribute to our understanding of the mechanisms by which the nervous system can regulate its responses to sensory input, which may ultimately result in new treatments for attentional disorders. [unreadable] [unreadable]